Cellulose is a principal component constituting plants and widely exists in nature. Because cellulose is a hardly decomposable high-molecular polysaccharide comprising a glucose recurring unit polymerized through a .beta.-1,4-glycoside bond, development of cellulase that splits the glycoside bond of cellulose to facilitate efficient extraction of the glucose unit is essential to effective utilization of the cellulosic biomass.
The terminology "cellulase" is a generic name of a group of enzymes catalyzing an enzymatic reaction system in which cellulose is decomposed into glucose, cellobiose or cellooligosaccharides. Cellulase includes species called C.sub.1 enzyme, C.sub.x enzyme, .beta.-glucosidase, exo-.beta.-glucanase, endo-.beta.-glucanase, cellobiase, etc. according to their mode of action, but the details have not yet been made clear.
Utilization of biomass by use of cellulase has been studied. In particular, the oil crisis in the '70s gave occasion for technological development of effective utilization of plant biomass for constantly supplying raw materials of energy, industrial material, food, etc.
Further, cellulase has been on the market as an industrial enzyme preparation and used as a main component of various products, such as detergents, fiber treating agents, additives for feed, and digestants. Therefore cellulase is expected to be of great use in practice.
Under the above situation, the inventors of the present invention have previously studied enzymatic saccharification of cellulose and succeeded in isolating a fungi, Acremonium cellulolyticus Y-94 (deposited in National Institute of Bioscience & Human Technology, Agency of Industrial Science and Technology (located at 1-3, Higashi 1-chome, Tsukuba-shi, Ibaraki-ken 305, JAPAN) under an international deposit number FERM BP-5826 pursuant to Budapest Treaty which was transferred from the original deposit (FERM P-6867 deposited on Jan. 12, 1983) on Feb. 19, 1997), which is capable of producing cellulase as reported in Japanese Patent Publication No. 43954/85.
The cellulase produced by this strain is a complex enzyme system (hereinafter sometimes referred to as a cellulase system) comprised mainly of three enzyme groups; C.sub.1 enzymes represented by avicellase or FPase, C.sub.x enzymes acting on carboxymethyl cellulose (CMC), and .beta.-glycosidase acting on cellooligosaccharides, such as cellobiose. Natural cellulose can be completely decomposed ultimately to glucose by these enzyme groups acting in harmony. The properties of the above-described cellulase system are described in the above-cited publication.
The cellulase system is characterized in that it has higher .beta.-glucosidase activity than the cellulase having the origin from so far isolated microorganisms belonging to the genus Trichoderma or Asperaillus, and is therefore more successful in decomposing cellulose to glucose. However, there is no information on the identification of the protein composing the cellulase system produced by Acremonium cellulolyticus Y-94, amino acid sequence of the protein, DNA sequence of the gene coding for the amino acid sequence, and the like.